You know the friend who drinks too much? Or calls you all the time with their problems ignoring yours? Or doesn’t know when to stop being provocative or silly? Or the friend who always gets into romantic relationships with the wrong people, or doesn’t notice when things have changed? Or says the wrong thing, lapses of cruelty, stupidity and pettiness, that hang in the air like a dark cloud? You either know such a friend or are such a friend, or both.

Mike Bartlett goes from a subject hardly anyone knows about (the intimate lives of the Royal family) to a subject everyone knows about: friendships, complicated and imperfect, meetings that – however protracted – remain unresolved, the moment you should have said something and you didn’t, or the moment you turned your back or closed your eyes and something terrible happened. He captures every day conversations (although I don’t remember the last time I used the word honorificabilitudinitatibus) and examines them with a magnifying glass. The hue turns darker and more grotesque, the tension summed up in a phone call you don’t want to answer.

Rachael Stirling (for once out of period clothes and away from period stories) is stunning. She has the indefatigable energy of a clown: persistently funny and bursting with wit and desperation, her character is as confused about herself as she is lucid about others. She perfectly captures that person who is the most remarkable in the room, but also the most annoying and the saddest. Clarity of vision doesn’t bring happiness. John Hollingworth is equally good: his everyday energy has richness and irresistible openness.

James Grieve directs with remarkable simplicity, which is to say a sharp eye for the moments the ordinary thickens with tension. Lucy Osborne’s set resists unnecessary flourishes: a black box is in turns a gallery and a pub, a home only needs a red wall and a ladder.

Something about the plainness of the set and the directness of the relationships reminded me of another Mike Bartlett play, Cock. In that play, it was romantic relationships under the microscope and in the fighting arena. An Intervention focuses on something more fragile: there are no rituals for starting a friendship, or ending it, or keeping it alive. Children have friendship rituals, until they get older and other relationships get more complicated. It turns out, no relationship is more complicated than a friendship. “I thought we were best friends” says one character. The other laughs: “Only small children have best [friends]”.